Surfactant-free NaBH4-mediated synthesis of colloidal gold nanoparticles in water at room temperature: fine size control for active nanocatalysts

Size control over nanomaterials obtained by simple, environmentally friendly and cost-efficient methods is a key achievement to make of the most the unique size-dependent properties of nanoparticles (NPs). For instance, size-controlled gold (Au) NPs are in high demand for electronic, optical, biomedical applications or catalysis. In several colloidal syntheses, size control is achieved by controlling pH, temperature and/or the ratio of various chemicals. Unfortunately, in most cases, energy-demanding and/or several high purity chemicals are required to achieve a fine and reproducible size control and/or active catalysts. Here, the unique features of a room temperature colloidal synthesis of gold nanoparticles simply obtained using only NaBH 4 and HAuCl 4 in water, without the need for any other chemicals, are exploited. Size control is achieved by tuning the NaBH 4 /Au molar ratio, where larger ratios lead to larger nanoparticles, and/or by controlling the purity of the water used for the synthesis where the use of an increasing amount of lower purity water leads to larger nanoparticles. Leveraging on the NaBH 4 /Au molar ratio and/or water purity leads to a fine size control in the range 5-20 nm. Colloidal dispersions stable for months are easily obtained. The optimization performed in 2 mL scales up easily by a factor x50 in volume. The prepared nanoparticles are readily active nanocatalysts suitable for size-effects study for instance for water treatment and energy conversion with the examples of 4-nitrophenol reduction and the ethanol oxidation reduction.